The truck pictured above is spreading crystals of the salt calcium chloride on a snowy road to prevent ice from forming. The crystals lower the freezing point of water so when the snow melts, it won’t turn to ice. Do you know why calcium chloride lowers the freezing point of water? The answer has to do with the type of compound that calcium chloride is. Calcium chloride is an ionic compound.

What Are Ionic Compounds?

All compounds form when atoms of different elements share or transfer electrons. Compounds in which electrons are transferred from one atom to another are called ionic
compounds
. In this type of compound, electrons actually move between the atoms, rather than being shared between them. When atoms give up or accept electrons in this way, they become charged particles called ions. The ions are held together by ionic bonds, which form an ionic compound. Ionic compounds generally form between elements that are metals and elements that are nonmetals. For example, the metal calcium (Ca) and the nonmetal chlorine (Cl) form the ionic compound calcium chloride (CaCl
2
). In this compound, there are two negative chloride ions for each positive calcium ion. Because the positive and negative charges cancel out, an ionic compound is neutral in charge.

Q:
Now can you explain why calcium chloride prevents ice from forming on a snowy road?

A:
If calcium chloride dissolves in water, it breaks down into its ions (Ca
2+
and Cl
-
). When water has ions dissolved in it, it has a lower freezing point. Pure water freezes at 0°C. With calcium and chloride ions dissolved in it, it won’t freeze unless the temperature reaches -29°C or lower.

Crystals

Many compounds form molecules, but ionic compounds form crystals instead. A
crystal
consists of many alternating positive and negative ions bonded together in a matrix. Look at the crystal of sodium chloride (NaCl) in the
Figure
below
. The sodium and chloride ions are attracted to each other because they are oppositely charged, so they form ionic bonds.

Sodium chloride crystals are cubic in shape. Other ionic compounds may have crystals with different shapes.

Naming Ionic Compounds

Ionic compounds are named for their positive and negative ions. The name of the positive ion always comes first, followed by the name of the negative ion. For example, positive sodium ions and negative chloride ions form the compound named sodium chloride. Similarly, positive calcium ions and negative chloride ions form the compound named calcium chloride.

Q:
What is the name of the ionic compound that is composed of positive barium ions and negative iodide ions?

A:
The compound is named barium iodide.

Properties of Ionic Compounds

The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those ionic bonds. As a result, ionic compounds are solids with high melting and boiling points. You can see the melting and boiling points of several different ionic compounds in the
Table
below
. To appreciate how high they are, consider that the melting and boiling points of water, which is
not
an ionic compound, are 0°C and 100°C, respectively.

Solid ionic compounds are poor conductors of electricity. The strong bonds between their oppositely charged ions lock them into place in the crystal. Therefore, the charged particles cannot move freely and carry electric current, which is a flow of charge. But all that changes when ionic compounds dissolve in water. When they dissolve, they separate into individual ions. The ions can move freely, so they can carry current. Dissolved ionic compounds are called electrolytes.

The rigid crystals of ionic compounds are brittle. They are more likely to break than bend when struck. As a result, ionic crystals tend to shatter easily. Try striking salt crystals with a hammer and you’ll find that they readily break into smaller pieces. You can learn more about the properties of ionic compounds by watching the video at this URL:

Uses of Ionic Compounds

Ionic compounds have many uses. Some are shown in the
Figure
below
. Many ionic compounds are used in industry. The human body needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions.

Summary

Ionic compounds consist of oppositely charged ions that are held together by ionic bonds. The opposite charges cancel out so ionic compounds have a net neutral charge. Ionic compounds form when metals transfer valence electrons to nonmetals.

Ionic compounds exist as crystals rather than molecules. A crystal consists of many alternating positive and negative ions bonded together in a matrix.

Ionic compounds are named for their positive metal ion first, followed by their negative nonmetal ion.

Ionic compounds are solids with high melting and boiling points. They are good conductors of electricity but only when dissolved in water. Their crystals are rigid and brittle.

Ionic compounds have many uses in industry. They are also needed by humans and other living things.

Vocabulary

crystal
: Rigid, lattice-like framework of many ions bonded together that is formed by some compounds such as table salt (NaCl).

ionic compound
: Compound that forms when oppositely charged metal and nonmetal ions are held together in a crystal by ionic bonds.

Practice

Watch the tutorial on ionic compounds at the following URL, and then answer the questions below.